Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, green, and blue emitting LEDs. The ratio of the intensities of the various colors sets the color of the light as perceived by a human observer.
One method of varying LED intensity is to vary the amount of time the LED is turned on. In such schemes, the LED is pulsed on a time scale that is too fast to be seen by a human observer. During each cycle, the LED is on for some fraction of the cycle time. Since the observer's eye integrates the light received over a time period that is long compared to the cycle time, the observer “sees” a light source whose intensity is proportional to the duty factor, i.e., the ratio of the time the LED is turned on to the time the LED is turned off. The intensity is a linear function of the duty factor, and hence, the control system is relatively simple. Alternatively, the intensity of the LED can be varied by varying the magnitude of current through the LEDs.
To provide very accurate color reproduction, LED light sources often use a sensor and feedback system. A light source is constructed from a combination of three LEDs that emit red, green, and blue light and three photodetectors that observe the light generated by the LED and adjust the duty factors of each LED to provide the precise color desired. The feedback circuit must generate measurements of the average light being generated at each color from the pulsing light signal that is produced by each LED. This average light signal is typically generated by utilizing a low-pass filter to filter the output of each photodetector. The low-pass filter introduces a delay in the feedback system that is sufficient to cause the feedback system to become unstable when the light source is switched between different colors in which one or more of the duty factors change by a large amount. While such instabilities can be reduced by utilizing complex active filters, this solution increases the cost and complexity of the light source.